A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An illumination system is configured to condition a radiation beam and between a reticle and a substrate there is a projection system for imaging an irradiated portion of a reticle onto a target portion of the substrate. The illumination system includes components for directing, shaping or controlling a projection beam of irradiation, and these components typically include refractive optics, reflective optics, and/or catadioptric systems, for example.
Generally, the projection system comprises an optical system to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system. The illumination system typically comprises adjustable elements for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of an intensity distribution upstream of a mask (in a pupil of the illumination system). A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be used to improve the processing parameters when an image of an illuminated object is projected onto a substrate.
Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular, the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. The smallest space and/or smallest width is referred to as the critical dimension (“CD”). In general, a single substrate will contain a network of adjacent target portions that are successively exposed.
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for isolated features and dense features may limit the process latitude (i.e. the available depth of focus in combination with an allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask having the same nominal CD will print differently depending on their pitch on the mask (i.e. the separation between adjacent features) due to pitch dependent diffraction effects. For example, a feature consisting of a line having a particular line width when in isolation (i.e. having a large pitch), will print differently from the same feature having the same line width when together with other lines of the same line width in a dense arrangement on the mask (i.e. having small pitch). Hence, when both dense and isolated features of CD are to be printed simultaneously, a pitch dependent variation of printed CD is observed. This phenomenon is called “iso-dense bias”, and is a particular problem in photolithographic techniques. Iso-dense bias is measured in nanometers and represents an important measure for practical characterization of lithography processes.
Conventional lithographic apparatus typically do not directly address the problem of iso-dense bias. It is the responsibility of users of the lithographic apparatus to attempt to compensate for the iso-dense bias by either changing optical parameters of the apparatus, such as the numerical aperture of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that differences in dimensions of printed isolated and dense features are minimized.
Generally, in a high volume manufacturing site different lithographic apparatus will be used for the same lithographic manufacturing process step to insure optimal exploitation of the machines, and consequently (in view of, for example, machine-to-machine differences) a variance and/or errors in CD may occur in the manufacturing process. Generally, the actual pitch dependency of such errors depends on the specific layout of the pattern and the features, aberrations occurring in the projection system of the lithographic apparatus in use, properties of the radiation sensitive layer on the substrate, and radiation beam properties such as illumination settings, and exposure dose of radiation energy. Therefore, given a pattern to be provided by a patterning device, and to be printed using a specific lithographic apparatus including a specific radiation source, one can identify data relating to iso-dense bias which are characteristic for that process, when executed on that lithographic system. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is the problem of mutually matching the corresponding different iso-dense bias characteristics, such as to reduce CD variations occurring in the manufacturing process.
A known technique to match an iso-dense bias characteristic of a machine (for a process whereby an annular illumination mode is used) to an iso-dense bias characteristic of another machine is to change the σ-outer and σ-inner settings, while maintaining the difference between the σ-outer and σ-inner settings (i.e. whilst maintaining the annular ring width of the illumination mode) of one of the two machines. The nominal σ-settings are chosen so as to optimize the process latitude (in particular, the depth of focus and the exposure latitude). In this approach, when the σ-settings are changed, the process latitude becomes smaller and may even become too small for practical use.
US Patent Application No. 2002/0048288A1 relates to an integrated circuit lithographic technique for bandwidth control of an electric discharge laser. The laser beam bandwidth is controlled to produce an effective beam spectrum having at least two spectral peaks in order to produce improved pattern resolution in resist. US Patent Application No. 2002/0048288A1 is incorporated herein by reference.
U.S. Pat. No. 5,303,002 relates to a method and apparatus for patterning a resist layer wherein a plurality of bands of radiation are used to provide an enhanced depth of focus. U.S. Pat. No. 5,303,002 is incorporated herein by reference.
U.S. Pat. No. 5,978,394 relates to a feedback arrangement for controlling a wavelength of a radiation beam. U.S. Pat. No. 5,978,394 is incorporated herein by reference.
One aspect of embodiments of the present invention provides a lithographic apparatus which allows iso-dense bias to be controlled.
A further aspect of embodiments of the present invention provides a method of controlling iso-dense bias in a lithographic apparatus.